ibcli: add new --extra-artifacts option with sbom support

This commit adds an option --extra-artifacts that can be
used to generate extra artifacts during the build or manifest
generation. Initially supported is `sbom` (but `manifest` is
planned too).

To use it run `--extra-artifacts=sbom` and it will generate
files like `centos-9-qcow2-x86_64.image-os.spdx.json` in
the output directory next to the generate runable artifact.

Closes: https://github.com/osbuild/image-builder-cli/issues/46

cmd/image-builder/build.go

@@ -1,9 +1,6 @@
 package main
 
 import (
-	"fmt"
-	"path/filepath"
-
 	"github.com/osbuild/images/pkg/imagefilter"
 	"github.com/osbuild/images/pkg/osbuild"
 )
@@ -12,9 +9,7 @@ func buildImage(res *imagefilter.Result, osbuildManifest []byte, osbuildStoreDir
 	// XXX: support output dir via commandline
 	// XXX2: support output filename via commandline (c.f.
 	//   https://github.com/osbuild/images/pull/1039)
-	outputDir := "."
-	buildName := fmt.Sprintf("%s-%s-%s", res.Distro.Name(), res.ImgType.Name(), res.Arch.Name())
-	jobOutputDir := filepath.Join(outputDir, buildName)
+	jobOutputDir := outputDirFor(res)
 
 	// XXX: support stremaing via images/pkg/osbuild/monitor.go
 	_, err := osbuild.RunOSBuild(osbuildManifest, osbuildStoreDir, jobOutputDir, res.ImgType.Exports(), nil, nil, false, osStderr)